-SPOF

I suppose you could deploy an HA cluster using either Ceph or StarWind VSAN HA storage technology. These solutions replicate data across cluster nodes, ensuring your VMs are highly available. They both fit Proxmox.

A Raspberry Pi with two network interfaces (one can be a USB-to-Ethernet or USB-to-WiFi adapter) can be set up as a failover router. Just write scripts to check the internet connectivity regularly and switch between connections as needed.